Acoustic waves, and at higher frequencies ultrasonic waves, can propagate along the surface of a material or in the bulk of a material, or a combination of the two. For example, acoustic waves that propagate along the surface of a material can be referred to as surface acoustic waves (SAWs) or surface ultrasonic waves (SUWs), and can have a bulk component. Acoustic waves, such as SAWs, have been used in a variety of commercial applications. For example, acoustic waves have been used for non-destructive evaluation (NDE) of bulk materials, single layer or multilayer films, and laminates. As another example, devices based on SAWs of sufficiently high frequency (e.g., 1 MHz or greater) can be used in radar and mm-wave signal processing As another example, chip scale devices are commercially available that do signal processing based on SAW technology. As yet another example, SAWs have been used in ultrasonic microscopy. In these and other practical applications, it can be useful to maintain constant propagation characteristics of the acoustic waves, e.g., based on proper choice of material or tuning the material properties so as to facilitate calibration. For example, acoustic waves obey the wave equation much like visible light. As such, minute changes in the local density of the material (e.g. index of the material for light waves) in which the acoustic wave is propagating can have an effect on one or both of the wave's phase front and the wave's Poynting vector (i.e. direction of propagation).